This is a bug that is a lot like the old trade spot bug.
Units getting stuck and become unmovable, only it is land units.
The way it works is this; If you over-build units in any city (a full stack in the city and overflow outside) the first turn the units are moveable but if you leave them they become stuck as a second stack in the city. This also happens if you gift a city to another faction with military access.
In any case when it happens you loose control over the units and they cannot be moved, only disbanded.
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